1. Field of the Invention
This invention resides in the field of propellant ingredients, and more specifically of bonding agents which allow hydroxy-terminated binders to cohesively interact with filler materials.
2. Brief Description of the Relevant Art
In the solid propellant industry, large quantities of propellant are required to be produced for space booster rockets. In recent years, this requirement of large quantities of propellant has taxed the capacity of state-of-the-art propellant manufacturing facilities.
One way to alleviate the problem is to shorten the batch mixing times required to produce solid propellants. The propellants currently are produced in a two-step process wherein all ingredients, including the liquid binder components (hereinafter referred to as the "prepolymer"), solid oxidizer particles, and bonding agent are mixed together for a period to fully mix the solid particles into the prepolymer. Only after this mixing stage is complete is a diisocyanate curative added to cure the propellant mix. When a hydroxy-terminated prepolymer is used with current polyol-type bonding agents, the first mixing stage is quite lengthy, as hydrogen bonding between the hydroxyl groups of the bonding agent and prepolymer create a viscous mixture. Also, when ammonium perchlorate (AP) is used as the solid oxidizer, chemisorption of the bonding agent to the AP particle surface evolves ammonia, requiring further vacuum mixing to remove the ammonia.
In order to reduce batch mixing times in propellants using AP as oxidizer and hydroxy-terminated prepolymers, aziridine-type bonding agents are sometimes used. The aziridine homopolymerizes to encapsulate the solid particles. This works well on acidic oxidizers such as AP, as the polymerization is acid catalyzed. In "clean" propellants, however, AP is used in combination with other oxidizers, such as NaNO.sub.3, which when combusted produce combustion products which neutralize the HCl evolved from AP combustion. NaNO.sub.3 is a neutral compound, and aziridines do not homopolymerize on it. As a result, aziridines do not perform well as bonding agents in mixed oxidizer systems.
This leaves the neutral, polyol-type bonding agents discussed above, but which require long batch mixing times. Examples are bis-(cyanoethyl)-dihydroxypropylamine, bis-(hydroxyethyl)-glycolamide, bis-(hydroxyethyl)-lactamide, and bis-(hydroxyethyl) dimethyl hydantoin. These bonding agents have at least one polar moiety which adheres to the surface of the oxidizer particles, while the hydroxyl groups react with the diisocyanate binder curative.
Another problem with the polyol-type bonding agents is that the diisocyanate curative reacts much more quickly with the hydroxyls of polybutadiene-type prepolymers than with the hydroxyls of the bonding agent. As the isocyanate groups are consumed by the reaction with the prepolymer, fewer and fewer are left to react with the bonding agent. The urethane shell around the oxidizer particles is not complete, leading to weak bonding of binder to oxidizer particles. Complete reaction of all binder hydroxyls is undesirable because this causes the binder to become overly cross-linked, increasing the modulus to a value such that the propellant is not useful. One solution to this problem is to pre-terminate (i.e., cap off) some of the binder hydroxyls with a monoisocyanate, thereby limiting the cross-linking of the binder. This however introduces another step in the propellant manufacturing process, which is generally undesirable for practical reasons such as requiring additional quality controls.